1. Field of the Invention
The present invention relates to three-dimensional image processing and, more particularly, to processing for generating a projection image from a three-dimensional image which includes an unintended display area.
2. Description of the Related Art
In the medical field, observation and diagnosis of projection images which are three-dimensional medical images, obtainable by CT systems, MRI systems, ultrasonic diagnostic systems, or the like, projected on desired projection planes are performed. In order to obtain such a projection image, image processing, in which a plurality of search points is determined in a three-dimensional medical image along a visual line passing from an arbitrary viewpoint to each projected pixel and, based on pixel values of the plurality of determined search points, pixel values of projected pixels are obtained with respect to each visual line, is performed. Such type of known image processing includes, for example, MIP (Maximum Intensity Projection) processing in which a maximum pixel value of search points is extracted with respect to each visual line and projected, MinIP (Minimum Intensity Projection) processing in which a minimum pixel value of search points is extracted with respect to each visual line and projected, and the like.
It is also known that, prior to generating a projection image, an unintended display area is determined in a three-dimensional medical image and image processing is performed on an intended display area. For example, a method that generates, from a three-dimensional cardiac image that includes a blood pool of aorta, which is not a display target, and coronary artery, which is a display target, an enhanced image that includes the coronary artery but not the blood pool through predetermined image processing and performs MIP processing on the enhanced image, thereby generating and displaying an image in which the coronary artery is enhanced without being influenced by the blood pool of unintended display target is proposed as described, for example, in Japanese Unexamined Patent Publication No. 11(1999)-242739.
When generating a projection image, the position of each search point on a visual line may possibly have a non-integer coordinate value in a coordinate system of three-dimensional image, depending on the positional relationship between the viewpoint and projection plane, so that there may be a case in which the pixel value of each pixel of a three-dimensional image can not be used directly. In such a case, it is necessary to calculate the pixel value of each search point by an interpolation operation based on the pixel values of pixels adjacent to each search point.
Now, when a three-dimensional image is divided into an intended display area and an unintended display area, a search point near the boundary of the two areas naturally lies in the intended display area but a pixel adjacent to the search point may possibly lie in the unintended display area. FIG. 4 shows such an example case, in which one pixel Q (x1, y2, z2) of eight pixels adjacent to a search point Pji (x, y, z) on a visual line Ej is a pixel in an unintended display area.
In such a case, if the pixel value of the search point Pji is calculated by an interpolation operation of adjacent pixel values, the result is influenced by the pixel value of adjacent pixel Q which is supposed to be an unintended display target. For example, when a pixel having a pixel value exceeding a pixel value threshold of 100 is assumed to be an unintended display target (non-display), the pixel value of adjacent pixel Q is 200, the pixel value of each of the other adjacent pixels (in the intended display area) is 100, and the search point Pji lies in the center of each of the adjacent pixels, the pixel value of the search point Pji obtained by the interpolation operation is, 100×(⅛)×7+200×(⅛)=112.5, which is greater than the threshold value representing the maximum value of display target pixel value, i.e., the value which should be determined as an unintended display pixel value. Here, if a projection image is generated by MIP processing, the pixel value of the search point is used on the visual line having this search point, whereby it appears as an artifact in the projection image.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide an apparatus and method capable of preventing an artifact arising from an unintended display area when generating a projection image produced by projected pixels of a three-dimensional image that includes an unintended display area. It is a further object of the invention to provide a computer readable recording medium on which is recorded a program for causing a computer to perform the method.